The present invention relates to a high speed tire uniformity testing device which is constructed for maximum stiffness and minimum mass for producing high resonant frequencies thereby assuring that no mechanical resonances exist within the measurement frequency range.
In the prior art, there are a number of tire testing machines of various kinds such as shown in U.S. Pat. Nos. 3,060,734, 3,206,973, 3,543,576, 3,546,936, 3,604,245 and 3,797,306. These devices typically include large frames that are required for moving the tire into position against a testing wheel. Such cumbersome mechanisms increase the cost of construction and make installation and servicing difficult. None of these devices provide an efficient, relatively lightweight and accurate tire testing apparatus that can be easily instrumented to provide for an analysis of all of the required loads and movements necessary for tire compliance testing.
Tire uniformity measurements are primarily associated with an assessment of the radial, fore/aft, and lateral force variations to be expected at the center of a tire rolling at a constant loaded radius. The need for tire uniformity measurements arose from the fact that the tire can be an exciter of vehicle vibrations. Previously, this sensitivity was mostly related to the first harmonic of the tire's non-uniformity. This first harmonic sensitivity feels very much like tire unbalance, having a once per tire revolution excitation rate.
The force variation signals from a tire can be decomposed into a series of sinusoidal components wherein the component having one cyclic variation per tire revolution being called the first harmonic, that with two complete cycles per tire revolution being called the second harmonic, etc. When one of these harmonics excites a resonant mode of vehicle vibration, a noticeable disturbance may be felt inside the vehicle. Since the first harmonic force typically has the largest peak-to-peak amplitude, that harmonic is usually the most noticeable. However, the higher harmonics become noticeable in the form of beating together, i.e., interfering and reinforcing one another. Nevertheless, typical tire uniformity measurements in the prior art have only included the first harmonic peak-to-peak values for the radial and lateral force variation signals from the tire since it was believed that low speed measurement of these quantities provided the necessary correlation to ride comfort ratings.
With the ever-increasing use of radial tires, which have lower resonant frequencies than bias ply tires, it has become important to measure higher harmonic excitations because of the disturbances caused by these higher harmonics. A problem exists, however, because prior devices are not able to readily assess the disturbances produced by the higher harmonics. Moreover, the smaller and lighter vehicles being produced today with unibody construction are generally more sensitive to these higher harmonics, thereby producing an even greater need for such measurements.
Therefore, it is a principal object of the present invention to provide a high speed tire uniformity testing device which will not resonate within the test frequency range of approximately 1-200 Hz that is required for measuring higher tire harmonics. It is preferred that the device include a very stiff but lightweight load frame so that all resonances therein are above 200 Hz with the load frame being easy to install, relocate and maintain.